sustainability
Review
Experimental Research on Primary and Secondary
Conversion Efficiencies in an Oscillating Water
Column-Type Wave Energy Converter
Tengen Murakami 1, *, Yasutaka Imai 1 , Shuichi Nagata 1 , Manabu Takao 2
and Toshiaki Setoguchi 1
1
2
*
Institute of Ocean Energy, Saga University, 1, Honjo, 840-8502 Saga, Japan; [email protected] (Y.I.);
[email protected] (S.N.); [email protected] (T.S.)
Department of Mechanical Engineering, Matsue National College of Technology, 14-4, Nishiikuma,
Matsue, 690-8518 Shimane, Japan; [email protected]
Correspondence: [email protected]; Tel.: +81-955-20-2190
Academic Editors: Diego Vicinanza and Mariano Buccino
Received: 29 June 2016; Accepted: 2 August 2016; Published: 5 August 2016
Abstract: For the practical application of a fixed oscillating water column (OWC)-type wave energy
converter, it is necessary to develop a design method which can consider the characteristics of
incident wave motion, the motion of the internal free surface affected in the structure such as a partly
submerged wall, the fluctuation of air pressure in an air chamber, and the rotation of the air turbine.
On the other hand, the impulse turbine as the secondary conversion device in the OWC unit is
expected to achieve high efficiency. In this paper, firstly, the steady air flow tests for a single-impulse
turbine were conducted to grasp the characteristics of the turbine without the effect of water waves.
Secondly, the 2-dimensional wave tank tests in regular waves for the performance evaluation of
the fixed OWC-type wave energy converter with the same impulse turbine were conducted to
obtain the data needed to make this design method. As the results, the effects of the air chamber
length and the guide vane’s setting angle on the primary and secondary conversion efficiencies are
clarified experimentally.
Keywords: wave energy converter; oscillating water column; primary conversion; secondary
conversion; impulse turbine
1. Introduction
For the next leap in power technologies to become a sustainable society, we are under obligation
not only to cope with the warming of the global environment but also to conserve the natural ecosystem
and coexist with nature. As for the renewable energy resources in the world, we can newly exploit
the mini-/micro-hydro, the wind, the solar and the ocean energy, etc. Wave energy which is one
of the renewable energies attracts attention as a promising resource that can reduce CO2 emissions.
Therefore, the wave energy converter (WEC) which converts wave power into electric power has been
developed all over the world and many types of WECs [1–3] such as the Oyster WEC as the movable
body type [4] and the Sea-wave Slot-cone Generator (SSG) WEC as the overtopping waves type [5]
are proposed.
There is an oscillating water column (OWC)-type wave energy converter as one of the WECs.
This device is composed of an air chamber, an air turbine and a generator and is considered to be safe
even under storm conditions. Many studies on this device [6–8] have been performed experimentally
and theoretically since the early 1970s.
Toyota et al. [9,10] carried out research with the aim of the practical application of the Backward
Bent Duct Buoy (BBDB), which is one of the OWC-type WECs. This device consists of an air chamber,
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an L-shaped duct, a buoyancy chamber and an air turbine. The BBDB has some advantages, which are
that the primary conversion efficiency is higher than that of other floating OWCs, and the mooring
cost is low because it is able to advance to the weather side by itself.
Takahashi et al. [11] designed a fixed pneumatic-type wave power extractor. This extractor is
a special type of concrete caisson, and it has an air chamber where wave power is extracted and is
converted into air power. Besides, Takahashi et al. [12] conducted the field test by installing the Wells
turbine and a generator. Lin et al. [13] installed the Savonius rotor turbine in an OWC. In this research,
turbine efficiency was evaluated while changing the wave period and turbine rotation speed. In the
results, a turbine conversion rate of about 20% was shown experimentally.
In the performance evaluation of the OWC-type WEC, it is necessary to consider the characteristics
of the incident wave motion, the motion of the internal free surface affected in the structure such as a
partly submerged wall, the fluctuation of air pressure in an air chamber, and the rotation of the air
turbine. However, most of the past studies were carried out by dividing the test into two steps of wave
tank tests and air turbine tests.
Liu et al. [14] simulated the performance of the OWC without the turbine numerically. In this
study, the nozzle effects of the chamber-duct system on the relative amplitudes of the inner free water
surface and air flow rate in the duct were investigated. Korde [15] proposed the reactive loading
circuit for the control of the natural frequency in the OWC, and it was shown that a high performance
associated with resonant oscillations could be obtained.
On the other hand, by means of the test rig having a piston-cylinder to generate the oscillating air
flow, the performance of the air turbine was evaluated in a series of studies. Setoguchi et al. [16,17]
reviewed a variety of experimental results concerning the performances of the Wells turbine and the
impulse turbine. Takao et al. [18] show that the impulse turbine with a fixed guide vane was superior
to the Wells turbine in the actual sea with irregular waves.
This paper discusses the primary and the secondary conversion efficiencies of the fixed OWC-type
WEC equipped with an impulse turbine. Firstly, the performance of the impulse turbine with fixed
guide vanes in the model OWC is confirmed by the steady air flow test without the effect of water
waves. Secondly, the effects of the air chamber length and the guide vane’s setting angle on the primary
and secondary conversion efficiencies are examined with the 2-dimensional wave tank tests in regular
waves, although there are other geometric parameters such as the curtain wall shape and the rotor
inlet/outlet angle having an impact on the efficiency.
2. Experimental Apparatus
The wave tank test was carried out using regular waves, though real sea waves are irregular,
because it is difficult to completely absorb the reflected wave with the wave generator in the case of
irregular waves. Besides, it seems that the test with regular waves is acceptable because one of the
research objectives is to confirm the highest value of efficiency while changing the wave length and
the rotational speed of the turbine. Figure 1 shows the arrangement of the experimental devices in
the 2-dimensional wave tank. This tank is 18.5 m long, 0.8 m wide and contains a 0.8 m water depth.
An absorbing wave generator was installed at the end of the tank and the model turbine was located
at the other end of the tank. Four wave height gauges (TS-DWG) produced by TECHNO SERVICE Co.
Ltd. (Kumamoto, Japan) were arranged in order to measure the amplitudes of the incident wave and
reflected wave accurately [19]. The wave data is fed into the computer through the analog-to-digital
converter (PCI-3165) from the Interface Corporation (Hiroshima, Japan). The incident wave height
was configured as high as possible in this wave tank, and the value was 0.1 m.
Figure 2 shows the model OWC-type wave energy converter with the impulse turbine. In the
experiments, the turbine was driven by the alternating-current synchronous motor (HG-JR73)
manufactured by the Mitsubishi Electric Corporation (Tokyo, Japan). The torque transducer (SS-005)
and the electromagnetic rotation detector (MP-981) produced by ONO SOKKI Co. Ltd. (Yokohama,
Japan) were located at the end of the turbine shaft. The basic chamber length is 0.7 m, the curtain wall
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depth is 0.1 m and the thickness of the curtain wall is 0.005 m. The schematic design of the air chamber
of 11
11
33 of
of numerical studies [20].
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was
conducted
in756
a series
Figure
1.
Arrangements
of
model
turbine
and
wave
height gauges
gauges in
in 2D
2D wave
wave tank.
tank.
Figure
Figure 1.
1. Arrangements
Arrangements of
of model
model turbine
turbine and
and wave
wave height
Figure
2. Model
Model
OWC
with
impulse
turbine.
Figure
Figure 2.
Model OWC
OWC with
with impulse
impulse turbine.
turbine.
Figure 33 indicates
indicates the locations
locations of
of the
the pressure
pressure gauge
gauge (AP-10S)
(AP-10S) and
and the
the wave
wave height
height gauges
gauges (FW(FWFigure
Figure 3 indicatesthe
the locations
of the
pressure gauge
(AP-10S) and
the wave height
gauges
H07
and
UD320)
made
by
the
KEYENCE
CORPORATION
(Osaka,
Japan)
at
the
top
of
the
air
H07
and and
UD320)
made
by the
KEYENCE
CORPORATION
(Osaka,
Japan)
at atthe
(FW-H07
UD320)
made
by the
KEYENCE
CORPORATION
(Osaka,
Japan)
thetop
topofofthe
the air
air
chamber. The
The rectangular
rectangular orifice
orifice of
of the
the air
air chamber
chamber is
is located
located at
at the
the center
center and
and three
three wave
wave height
height
chamber.
chamber. The rectangular orifice of the air chamber is located at the center and three wave height
gauges were
were set up
up at the
the starboard side
side and portside,
portside, respectively. The
The data of
of waves, the
the turbine
gauges
gauges were set
set up at
at the starboard
starboard side and
and portside, respectively.
respectively. The data
data of waves,
waves, the turbine
turbine
output torque,
torque, the
the turbine
turbine rotational
rotational speed,
speed, the
the pressure
pressure and
and the
the water
water surface
surface elevation
elevation in
in the
the air
output
output torque, the turbine rotational speed, the pressure and the water surface elevation in the air
air
chamber
were
measured
simultaneously.
The
sampling
frequency
is
50
Hz,
and
the
30
s
data
after
chamber
chamber were
were measured
measured simultaneously.
simultaneously. The
The sampling
sampling frequency
frequency is
is 50
50 Hz,
Hz, and
and the
the 30
30 ss data
data after
after
starting the
the wave generator
generator was appropriately
appropriately neglected
neglected to
to evaluate
evaluate the
the OWC
OWC performance.
performance.
starting
starting the wave
wave generator was
was appropriately neglected
to evaluate
the OWC
performance.
Figure 44 shows
shows the basic
basic configuration of
of the rotor
rotor and the
the fixed guide
guide vanes. The
The rotor consists
consists
Figure
Figure 4 shows the
the basic configuration
configuration of the
the rotor and
and the fixed
fixed guide vanes.
vanes. The rotor
rotor consists
of
the
2D
blades.
The
numbers
of
the
rotor
blades
and
the
single-stage
guide
vanes
are 26,
26,
of
the2D
2Dblades.
blades.
numbers
ofrotor
the blades
rotor blades
the single-stage
guide
vanes
are
of the
TheThe
numbers
of the
and theand
single-stage
guide vanes
are 26,
respectively.
respectively.
The
inlet/outlet
angle
γ
of
the
rotor
is
60
degrees
and
the
setting
angle
θ
of
the
guide
respectively.
Theangle
inlet/outlet
γ 60
of degrees
the rotor
is 60
and the
angle
θ of
thedegrees.
guide
The inlet/outlet
γ of the angle
rotor is
and
thedegrees
setting angle
θ ofsetting
the guide
vane
is 30
vane
is
30
degrees.
The
turbine
configuration
was
adopted
on
the
basis
of
the
air
turbine
test
results
vane
is 30 degrees. The turbine configuration was adopted on the basis of the air turbine test results
The turbine configuration was adopted on the basis of the air turbine test results [16].
[16].
[16].
The inner
inner diameter
diameter D
D of
of the
the casing
casing is
is 170
170 mm,
mm, the
the hub
hub ratio
ratio νν is
is 0.7
0.7 and
and the
the clearance
clearance between
between
The
the
rotor
blade
tip
and
the
casing
is
0.3
mm.
The
inner
diameter
D
of
the
turbine
casing
was
calculated
the rotor blade tip and the casing is 0.3 mm. The inner diameter D of the turbine casing was calculated
by the
the following
following equations.
equations.
by
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44 of
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11
Figure
height gauges
gauges at
at top
top of
of air
air chamber.
chamber.
Figure 3.
3. Positions
Positions of
of pressure
pressure gauge
chamber.
gauge and
and wave
wave height
Figure 4.
Configuration of rotor
and guide
vane.
Figure
Figure 4.
4. Configuration
Configuration of
of rotor
rotor and
and guide
guide vane.
vane.
The inner diameter D of the casing is 170 =
mm, the
the
// hub ratio ν is 0.7 and the clearance between(1)
=
(1)
rotor blade tip and the casing is 0.3 mm. The inner diameter D of the turbine casing was calculated by
=
the following equations.
= 0.5
0.5
(2)
(2)
ε “ S N {S
(1)
=
0.5
(3)
= 0.5
(3)
∆p N “ 0.5C p ρv2N
(2)
=
))
2
= (1
(1 ∆p )(2.75
)(2.75
1.56
“ 0.5ψρv1.56
a
(4)
(3)
(4)
where
denote
the
ratio
of
Cp v
“
´ εq p2.75
´ 1.56εq
where ε,
ε, S
SNN,, S,
S, Δp
ΔpNN,, C
Cpp,, ρ,
ρ, vvNN,, Δp,
Δp, ψ
ψ and
and
vaa p1
denote
the nozzle
nozzle
ratio (=1/100),
(=1/100), the
the cross-section
cross-section area
area(4)
of
the
nozzle,
the
cross-section
area
of
the
air
chamber,
the
total
pressure
difference
at
the
nozzle,
the
the
nozzle,
the
cross-section
area
of
the
air
chamber,
the
total
pressure
difference
at
the
nozzle,
the
where ε, SN , S, ∆pN , Cp , ρ, vN , ∆p, ψ and va denote the nozzle ratio (=1/100), the cross-section area of
pressure
pressure loss
loss coefficient
coefficient of
of the
the nozzle,
nozzle, the
the air
air density,
density, the
the axial
axial flow
flow velocity
velocity in
in the
the nozzle,
nozzle, the
the total
total
the nozzle, the cross-section area of the air chamber, the total pressure difference at the nozzle, the
pressure
drop
at
the
turbine,
the
turbine
pressure
coefficient,
and
the
axial
flow
velocity
in
pressure drop at the turbine, the turbine pressure coefficient, and the axial flow velocity in the
the
pressure loss coefficient of the nozzle, the air density, the axial flow velocity in the nozzle, the total
turbine.
turbine.
pressure drop at the turbine, the turbine pressure coefficient, and the axial flow velocity in the turbine.
Firstly,
Firstly, the
the pressure
pressure loss
loss coefficient
coefficient of
of C
Cpp containing
containing the
the S
S corresponding
corresponding to
to the
the product
product of
of the
the
Firstly, the pressure loss coefficient of Cp containing the S corresponding to the product of the
tank
width
and
the
air
chamber
length
can
be
calculated
by
Equation
(4),
and
the
turbine
pressure
tank width and the air chamber length can be calculated by Equation (4), and the turbine pressure
tank width and the air chamber length can be calculated by Equation (4), and the turbine pressure
coefficient
coefficient of
of ψ
ψ is
is determined
determined by
by Equation
Equation (3)
(3) which
which is
is composed
composed of
of the
the total
total pressure
pressure drop
drop Δp
Δp and
and
coefficient of ψ is determined by Equation (3) which is composed of the total pressure drop ∆p and
the
axial
flow
velocity
v
a. The Δp and va are the experimental results obtained by the preliminary
the axial flow velocity va. The Δp and va are the experimental results obtained by the preliminary
the axial flow velocity va . The ∆p and va are the experimental results obtained by the preliminary
turbine
turbine test
test [18].
[18]. Moreover,
Moreover, the
the total
total pressure
pressure difference
difference at
at the
the nozzle
nozzle is
is equal
equal to
to the
the total
total pressure
pressure
turbine test [18]. Moreover, the total pressure difference at the nozzle is equal to the total pressure
drop
drop at
at the
the turbine
turbine (Δp
(ΔpNN == Δp),
Δp), and
and the
the flow
flow rate
rate in
in the
the turbine
turbine is
is the
the same
same as
as in
in the
the case
case of
of the
the nozzle
nozzle
drop at the turbine (∆pN = ∆p), and the flow rate in the turbine is the same as in the case of the nozzle
(Q
=
v
aST = vNSN). These relations result in the following equation.
(Q = vaST = vNSN). These relations result in the following equation.
(Q = va ST = vN SN ). These relations result in the following equation.
.
// =
= (( // )) .
(5)
(5)
`
˘0.5
S
{S
“
ψ{C
(5)
p
T
N
where
is the
the flow
flow passage
passage area
area of
of the
the turbine.
turbine. Therefore,
Therefore, the
the inner
inner diameter
diameter D
D as
as the
the hub
hub ratio
ratio νν ==
where S
STT is
0.7
0.7 is
is derived
derived from
from the
the known
known S
SNN,, ψ
ψ and
and C
Cpp..
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where ST is the flow passage area of the turbine. Therefore, the inner diameter D as the hub ratio
2016, 8, from
756 the known SN , ψ and Cp .
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νSustainability
= 0.7 is derived
Test
3.3. Steady Air Flow Test
Tograsp
graspthe
theturbine’s
turbine’sperformance,
performance,we
weconducted
conductedaasteady
steadyflow
flowtest.
test. The
The bottom
bottom of
of the
the air
air
To
chamber
in
Figure
2
was
closed
and
the
steady
air
flow
was
generated
by
a
centrifugal
fan.
Figure
chamber in Figure 2 was closed and the steady air flow was generated by a centrifugal fan. Figure 55
shows the
theexperimental
experimental results
resultsof
ofthe
theturbine
turbineperformance
performancewhere
whereηη2 2 isisthe
thesecondary
secondaryconversion
conversion
shows
efficiency,CCTT isis the
the torque
torque coefficient,
coefficient, CCAA is
is the
the input
input coefficient
coefficient and
and the
the abscissa
abscissa φφ isis the
the flow
flow
efficiency,
coefficient.
The
definitions
of
these
parameters
are
as
follows:
coefficient. The definitions of these parameters are as follows:
20
0.6
D = 0.30 m
D = 0.17 m
15
C
T
2
0.4
0.2
10
5
D = 0.30 m
D = 0.17 m
0
0
0
1
2
3
0
4
1
2
3
4


(a)
(b)
20
D = 0.30 m
D = 0.17 m
C
A
15
10
5
0
0
1
2
3
4

(c)
Figure5.5.Turbine
Turbineperformances
performanceson
onsteady
steadyair
airflow
flowconditions.
conditions.(a)
(a)Secondary
Secondaryconversion
conversion efficiency;
efficiency;
Figure
(b)Torque
Torquecoefficient;
coefficient;(c)
(c)Input
Inputcoefficient.
coefficient.
(b)
=
/(
)
η2 “ T0 ω{ p∆pQq
”
´
¯
ı
CT =
“ T0/[0.5
{ 0.5ρ( v2a+` U)2 ST] r
”
´
¯
ı
/[0.5
CA =
“ ∆pQ{
0.5ρ( v2a+` U)2 S]T r
(6)
(6)
φ=“ v a/{U
(9)
(9)
(7)
(7)
(8)
(8)
where
whereTT00,,ω,
ω, Q
Q and
and U
U denote
denote the
the turbine
turbine output
output torque,
torque, the
the angular
angularvelocity
velocityof
ofthe
therotor,
rotor,the
theflow
flowrate
rate
and
the
circumferential
velocity
at
mean
radius
r
[=
D(1
+
ν)/4],
respectively.
The
flow
rate
Q
was
and the circumferential velocity at mean radius r [= D(1 + ν)/4], respectively. The flow rate Q was
measured
measuredby
bythe
thevelocity
velocitymeter
meter(KANOMAX
(KANOMAX6501)
6501)atatthe
thefan
fanduct.
duct.
In
Figure
5,
D
=
0.30
m
denotes
the
result
of
the
conventional
In Figure 5, D = 0.30 m denotes the result of the conventionalturbine
turbinetest
testwithout
withoutthe
theair
airchamber
chamber
and
the
elbow
measured
by
Takao
et
al.
[16,18].
The
turbine
with
D
=
0.17
m
prepared
for
this
study
is
and the elbow measured by Takao et al. [16,18]. The turbine with D = 0.17 m prepared for this
study
geometrically
similar
to the
D =of0.30
In the
of Dcase
= 0.17
is noticed
the maximum
is geometrically
similar
tocase
the of
case
D =m.0.30
m.case
In the
of m,
D =it 0.17
m, it that
is noticed
that the
ηmaximum
which
is
as
high
as
the
case
of
D
=
0.30
m,
although
the
values
are
low
at the
2 of 0.48 isηachieved
2 of 0.48 is achieved which is as high as the case of D = 0.30 m, although the values
are
high
flow
rate
due
to
the
increase
of
C
.
A
low at the high flow rate due to the increase
of CA.
4. Oscillating Air Flow Tests by Water Waves
4.1. Effect of Air Chamber Length
To know the effects of the air chamber length L on the primary and secondary conversion
efficiencies, the length was changed from L = 0.7 m to 0.5 m and 0.9 m as shown in Figure 6.
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4. Oscillating Air Flow Tests by Water Waves
4.1. Effect of Air Chamber Length
To know the effects of the air chamber length L on the primary and secondary conversion
efficiencies,
the length
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Figure
Figure6.6. Air
Air chamber
chamber configuration.
configuration.
η=
1 “ Pair
/ {Pwave
(10)
(10)
(11)
(11)
(12)
η2 “ Ptorque {Pair
=
/
η “ Ptorque {Pwave
/
=
where Pair is the time-averaged power of the compressed air, Pwave is the time-averaged power of(12)
the
incident
and
Ptorque is the turbine
output.
The definitions
these
are aspower
follows:
where Pwave
air is the
time-averaged
power of
the compressed
air, Pof
wave
is theparameters
time-averaged
of the
incident wave and Ptorque is the turbine output. The
ż definitions of these parameters are as follows:
Bζ
S T
p ptq
dt
(13)
Pair “=
(
)
(13)
T 0
Bt
= 0.5ρ
0.5w gζ i2 Cg W
Pwave “
ż
11 T
Ptorque “=
T0 ωdt
T 0
(14)
(14)
(15)
(15)
1, 2, 
where
and W
W denote
denote the
the period
period of the incident wave, the pressure,
g, ζζii, Cgg and
pressure, the
the six
six averaged
averaged
whereT,
T,p,p,ζ,ζ,ρρww,, g,
water
water level
level in
in the
the air
air chamber,
chamber,the
thewater
waterdensity,
density,the
thegravitational
gravitationalacceleration,
acceleration, the
the incident
incident wave
wave
amplitude,
amplitude,the
thegroup
groupvelocity
velocityand
andthe
thechamber
chamberwidth,
width,respectively.
respectively.
Figure
Figure77shows
showsthe
thevariations
variationsof
ofthe
theprimary
primaryconversion
conversionefficiency
efficiencyηη11,, the
the secondary
secondary conversion
conversion
efficiency
efficiency ηη22 and
and the
the generating
generating efficiency
efficiency ηη due
due to
to the
the rotational
rotational speed
speed N
N of
ofthe
theturbine.
turbine. The
The wave
wave
length
lengthλλwas
waskept
keptconstant
constantatatabout
aboutλ/L
λ/L==6.3.
6.3. In
In the
thethree
threecases
casesof
ofLL== 0.5
0.5 m,
m, 0.7
0.7 m
m and
and 0.9
0.9 m,
m, the
the wave
wave
periods
periodscorresponding
correspondingto
toabout
aboutλ/L
λ/L== 6.3
6.3 are
areTT == 1.50
1.50 s,
s, 1.87
1.87 ss and
and 2.30
2.30 s,
s, respectively.
respectively. The
The generating
generating
efficiency
efficiency ηηisisdefined
definedas
asthe
theproduct
productofofηη1 1and
andηη22.. The
Theefficiencies
efficienciesηη11,, ηη22 and ηη are
are calculated
calculated by
by the
the
following
followingequations.
equations.
As shown in Figure 7, the maximum η was achieved at about N = 700 rpm in all three cases
of L = 0.5 m, 0.7 m and 0.9 m, and0.8the maximum
value
of 0.28 was shown in the case of L = 0.7 m.
L = 0.9 m
L = 0.5 m
L = 0.7in
m the case of L = 0.5 m because the η was decreased.
Meanwhile, the maximum η was decreased
1
The remarkable difference of the maximum
η 2 between the three cases was not seen.
0.6
1
0.4
0.2
0
2

Figure 7 shows the variations of the primary conversion efficiency η1, the secondary conversion
efficiency η2 and the generating efficiency η due to the rotational speed N of the turbine. The wave
length λ was kept constant at about λ/L = 6.3. In the three cases of L = 0.5 m, 0.7 m and 0.9 m, the wave
periods corresponding to about λ/L = 6.3 are T = 1.50 s, 1.87 s and 2.30 s, respectively. The generating
efficiency η is defined as the product of η1 and η2. The efficiencies η1, η2 and η are calculated by the
Sustainability 2016, 8, 756
7 of 11
following equations.
0.8
L = 0.9 m
L = 0.7 m
1, 2, 
0.6
L = 0.5 m
1
0.4
2
0.2

Sustainability 2016, 8, 756
7 of 11
0
0
500
1000
1500
2000
As shown in Figure 7, the maximum η was achieved at about N = 700 rpm in all three cases of L
N rpm
= 0.5 m, 0.7 m and 0.9 m, and the maximum value of 0.28 was shown in the case of L = 0.7 m.
Meanwhile, the
maximum
η was
decreased
in the case
of L = 0.5
mgenerating
because the
η1 was decreased. The
Figure
7.
conversion,
secondary
conversion
and
efficiencies.
Figure
7.Primary
Primary
conversion,
secondary
conversion
and
generating
efficiencies.
remarkable difference of the maximum η2 between the three cases was not seen.
Figure
airair
flow
test
and
thethe
oscillating
air air
flow
testtest
on
Figure 88shows
showsaacomparison
comparisonbetween
betweenthe
thesteady
steady
flow
test
and
oscillating
flow
the
turbine
performance.
The The
abscissa
is theisflow
coefficient
φ. It is
that the
trends
in the in
three
on the
turbine
performance.
abscissa
the flow
coefficient
ϕ.noticed
It is noticed
that
the trends
the
cases
of
the
oscillating
air
flow
tests
are
similar
to
the
one
of
the
steady
air
flow
test,
although
three cases of the oscillating air flow tests are similar to the one of the steady air flow test, although the
the
maximum
η
2 = 0.45 in the case of the wave tank test is slightly low because of the CA increase.
maximum η = 0.45 in the case of the wave tank test is slightly low because of the C increase.
A
2
0.6
20
15
C
T
2
0.4
0.2
Steady flow test
L = 0.9 m
Steady flow test
L = 0.7 m
L = 0.9 m
L = 0.5 m
10
5
L = 0.7 m
L = 0.5 m
0
0
0
1
2
3
4
0
1
2

3
4

(a)
(b)
20
Steady flow test
L = 0.9 m
C
A
15
L = 0.7 m
L = 0.5 m
10
5
0
0
1
2
3
4

(c)
Figure
steady
airair
flow
and
oscillating
air air
flow
on turbine
performance.
(a)
Figure 8.
8. Comparison
Comparisonbetween
between
steady
flow
and
oscillating
flow
on turbine
performance.
Secondary
conversion
efficiency;
(b)
Torque
coefficient;
(c)
Input
coefficient.
(a) Secondary conversion efficiency; (b) Torque coefficient; (c) Input coefficient.
Next, the incident wave length λ was changed while keeping the rotational speed N = 700 rpm
Next, the incident wave length λ was changed while keeping the rotational speed N = 700 rpm
to make clear the maximum efficiencies derived from the configuration of the above model OWC.
to make clear the maximum efficiencies derived from the configuration of the above model OWC.
The wave periods are T = 1.00 s, 1.15 s, 1.30 s, 1.41 s, 1.50 s, 1.56 s, 1.65 s, 1.73 s, 1.87 s, 2.03 s, 2.30 s
The wave periods are T = 1.00 s, 1.15 s, 1.30 s, 1.41 s, 1.50 s, 1.56 s, 1.65 s, 1.73 s, 1.87 s, 2.03 s, 2.30 s and
and 2.63 s. Figure 9 shows the variations of the efficiencies due to the λ/L. The λ/L as the measurement
2.63 s. Figure 9 shows the variations of the efficiencies due to the λ/L. The λ/L as the measurement
points become small with the increase of L because of the same wave period T, as shown in the cases
points become small with the increase of L because of the same wave period T, as shown in the cases
L = 0.7 m and 0.9 m. In all three cases of L = 0.5 m, 0.7 m and 0.9 m, the maximum η2 is almost the
L = 0.7 m and 0.9 m. In all three cases of L = 0.5 m, 0.7 m and 0.9 m, the maximum η 2 is almost the
same value. On the other hand, the maximum η of L = 0.5 m was decreased due to the deterioration
same value. On the other hand, the maximum η of L = 0.5 m was decreased due to the deterioration of
of η1. Besides, the peaks of η1 appeared at λ/L = 6.3 and 4.4 in the cases of L = 0.7 m and 0.9 m, and the
value was η1 = 0.63 in both cases.
Figure 10 is a comparison of the reflection coefficient kr (reflected wave amplitude/incident wave
amplitude) in the three cases of L = 0.5 m, 0.7 m and 0.9 m. The values in the cases of L = 0.7 m and
0.9 m became low at λ/L = 6.3 and 4.4, respectively. In addition, in the case of L = 0.5 m, the kr at λ/L =
Sustainability 2016, 8, 756
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η 1 . Besides, the peaks of η 1 appeared at λ/L = 6.3 and 4.4 in the cases of L = 0.7 m and 0.9 m, and the
8 of 11
both cases.
Sustainability
756 in
value was 2016,
η 1 =8,0.63
0.8
L = 0.9 m
L = 0.7 m
L = 0.5 m
Sustainability 2016, 8, 756
1, 2, 
0.6
1
8 of 11
0.4
2
0.8
0.2
L = 0.9 m
L = 0.7 m
L = 0.5 m

1, 2, 
0.6
0
1
0
5
0.4
10
15
 /L
2
Figure
Figure 9.
9. Change
Changein
inefficiencies
efficiencies at
at time-averaged
time-averaged rotational
rotational speed
speed N
N ==700
700rpm.
rpm.
0.2

Figure 10 is a comparison of the reflection coefficient
kr (reflected wave amplitude/incident wave
L = 0.9 m
L
=
0.7
m
0
amplitude) in the three cases of L = 0.5 m, 0.7 m and
0.9 m. The values in the cases of L = 0.7 m and
5 L = 0.5 m
10
15
0.6 0
0.9 m became low at λ/L = 6.3 and 4.4, respectively. In addition, in the case of L = 0.5 m, the kr at
 /L
λ/L = 8.9, giving the maximum η 1 which is high compared with the above values of L = 0.7 m and
0.3
Figureof
9. kChange
inthe
efficiencies
time-averaged
rotational
speed
N shown
= 700 rpm.
0.9 m. This increase
decreaseatof
η 1 in the case
of L = 0.5
m as
in Figure 9.
r causes
kr
0.9
0.9
0
0
5
10
15
kr
0.6
L = 0.9 m
L = 0.7 m
L
= /L
0.5 m
Figure 10. Variation of reflection coefficient due to chamber length.
0.3
4.2. Effect of Setting Angle of the Guide Vane
0
1, 2, 
The θ was varied from the base
case of5 30.0 degrees
to 37.5 15degrees and 22.5 degrees to
0
10
investigate the effects of the setting angle θ of the
 /Lguide vane on the primary and secondary
conversion efficiencies.
Figureof
10.rotor
Variation
coefficient
to chamber
length.
Figure
10.
Variation
of reflection
Besides, the number
blades
was changed
fromdue
26 to
30 in order
to suppress the rotor–
guide vane interaction that causes the noise and the vibration based on the same number of rotor
4.2. Effect
Settingvanes,
Anglebut
of the
blades
andofguide
theGuide
effectVane
is not clarified in this paper. In Figure 11, the efficiencies are
compared
thefrom
three
θ = 22.5
degrees,
30.0
degrees
and
37.5
degrees.
The
chamber
The θθbetween
wasvaried
varied
from
the
case
ofdegrees
30.0 degrees
to 37.5
degrees
and 22.5
to
was
thecases
basebase
case
of 30.0
to 37.5
degrees
and
22.5
degrees
to degrees
investigate
length
L
is
0.7
m.
The
ratio
λ/L
was
kept
constant
at
6.3,
corresponding
to
the
wave
period
T
=
1.87
s.
investigate
the
effects
of
the
setting
angle
θ
of
the
guide
vane
on
the
primary
and
secondary
the effects of the setting angle θ of the guide vane on the primary and secondary conversion efficiencies.
As
shown
inefficiencies.
Figure
11, theofeffect
the θwas
on the
η1 is small
Norder
= 700 to
rpm
givingthe
therotor–guide
maximum
conversion
Besides,
the number
rotor of
blades
changed
from at
26 about
to 30 in
suppress
η.
Besides,
thethe
maximum
value
of
ηblades
= and
0.28 was
is obtained
atbased
θ = 30.0
degrees.
Besides,
number
ofthe
rotor
changed
from
26
in order
to suppress
the rotor–
vane
interaction
that
causes
noise
the
vibration
on to
the30same
number
of rotor blades
and
guide vanes,
vane interaction
that
causes
the
noise
and
the
vibration
based
on
the
same
number
of
rotor
but the effect is not clarified in this paper. In Figure 11, the efficiencies are compared
0.8
blades
and
thedegrees,
effect is30.0
not degrees
clarified
in this
In The
Figure
11, thelength
efficiencies
between
theguide
three vanes,
cases θ but
= 22.5
and
37.5paper.
degrees.
chamber
L is 0.7are
m.
 = 37.5
deg.

= 30.0degrees,
deg.
compared
between
the
three
cases
θ
=
22.5
30.0
degrees
and
37.5
degrees.
The
chamber
The ratio λ/L was kept constant at 6.3, corresponding
to the wave period T = 1.87 s. As shown in
 = 22.5 deg.
length
L isthe
0.7effect
m. The
was
constant
at 6.3,
the
wave period
T = 1.87the
s.
Figure 11,
of ratio
the θ λ/L
on the
is small
at about
N =corresponding
700 rpm givingtothe
maximum
η. Besides,
0.6η 1kept
1 θaton
As
shown in
Figure
the effect
of the
η1 degrees.
is small at about N = 700 rpm giving the maximum
maximum
value
of η11,
= 0.28
is obtained
θ =the
30.0
η. Besides, the maximum value of η = 0.28 is obtained at θ = 30.0 degrees.
0.4
2
0.8
0.2

1, 2, 
0.6
0
0.4
1
0
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
 = 60 deg. Zg = 26
Zr = 30
500
2
1000
1500
2000
N rpm
Figure 11. Effect of setting angle of guide vane on efficiencies.
0.2

guide vane interaction that causes the noise and the vibration based on the same number of rotor
blades and guide vanes, but the effect is not clarified in this paper. In Figure 11, the efficiencies are
compared between the three cases θ = 22.5 degrees, 30.0 degrees and 37.5 degrees. The chamber
length L is 0.7 m. The ratio λ/L was kept constant at 6.3, corresponding to the wave period T = 1.87 s.
As shown in Figure 11, the effect of the θ on the η1 is small at about N = 700 rpm giving the maximum
Sustainability 2016, 8, 756
9 of 11
η. Besides, the maximum value of η = 0.28 is obtained at θ = 30.0 degrees.
0.8
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
1, 2, 
0.6
1
0.4
2
0.2
0

 = 60 deg.
Zr = 30
0
500
Zg = 26
1000
1500
2000
N rpm
Figure
Figure11.
11.Effect
Effectof
ofsetting
settingangle
angleof
ofguide
guide vane
vane on
on efficiencies.
efficiencies.
Sustainability 2016, 8, 756
9 of 11
Figure 12
12shows
showsthe
the
amplitudes
of the
pressure
andwater
the water
elevation
the air
amplitudes
of the
pressure
and the
surfacesurface
elevation
in the airinchamber.
chamber.
It seems
that theincreases
pressure at
increases
smaller
based
the dragMeanwhile,
increase. Meanwhile,
the
It seems that
the pressure
smaller at
θ based
onθthe
dragonincrease.
the amplitude
amplitude
ofsurface
the water
surfacedecreases
elevationinversely
decreaseswith
inversely
with the
pressure increase.
of the water
elevation
the pressure
increase.
1.5
1.5
 = 60 deg. Zg = 26
 = 60 deg. Zg = 26
Zr = 30
Zr = 30
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
 /
i
1
w
P/ g
i
1
0.5
0.5
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
0
0
500
1000
1500
0
2000
0
500
1000
N rpm
1500
2000
N rpm
(a)
(b)
Figure
and
water
surface
elevation.
(a) Pressure
amplitude;
(b)
Figure 12.
12. Changes
Changesininamplitudes
amplitudesofofpressure
pressure
and
water
surface
elevation.
(a) Pressure
amplitude;
Water
surface
elevation.
(b) Water surface elevation.
Figure 13 shows the turbine performance. In Figure 13a, the maximum η2 of 0.48 was obtained
Figure 13 shows the turbine performance. In Figure 13a, the maximum η of 0.48 was obtained
at φ = 0.71 in the case of θ = 30.0 degrees. On the other hand, in both cases θ =237.5 degrees and 22.5
at φ = 0.71 in the case of θ = 30.0 degrees. On the other hand, in both cases θ = 37.5 degrees and
degrees, the maximum η2 was decreased slightly. In addition, the flow coefficient giving the
22.5 degrees, the maximum η was decreased slightly. In addition, the flow coefficient giving the
maximum η2 became small by 2the reduction of the θ from 30.0 degrees to 22.5 degrees. Furthermore,
maximum η became small by the reduction of the θ from 30.0 degrees to 22.5 degrees. Furthermore,
in the case of2 θ = 22.5 degrees, the CA increased markedly at around φ = 0.45, giving the maximum η2
in the case of θ = 22.5 degrees, the CA increased markedly at around φ = 0.45, giving the maximum
compared to the other two cases as shown
in Figure 13c. This fact corresponds to the pressure rise in
η 2 compared to the other two cases as shown in Figure 13c. This fact corresponds to the pressure
Figure 12a. In the case of θ = 22.5 degrees, along with the pressure rise, the turbine output torque
rise in Figure 12a. In the case of θ = 22.5 degrees, along with the pressure rise, the turbine output
becomes higher at a small flow rate due to the increase of the whirl velocity of the flow from the
torque becomes higher at a small flow rate due to the increase of the whirl velocity of the flow from
upstream guide vane, as shown in Figure 13b. This is the reason why the peak of η2 appeared in the
the upstream guide vane, as shown in Figure 13b. This is the reason why the peak of η 2 appeared in
small flow rate in the case θ = 22.5 degrees.
the small flow rate in the case θ = 22.5 degrees.
0.6
20
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
15
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
C
T
2
0.4
 = 60 deg. Zg = 26
Zr = 30
0.2
10
5
 = 60 deg. Zg = 26
Z = 30
r
0
0
0
1
2
3
4
0
1
2


(a)
(b)
20
15
 = 60 deg. Zg = 26
Zr = 30
3
4
in the case of θ = 22.5 degrees, the CA increased markedly at around φ = 0.45, giving the maximum η2
compared to the other two cases as shown in Figure 13c. This fact corresponds to the pressure rise in
Figure 12a. In the case of θ = 22.5 degrees, along with the pressure rise, the turbine output torque
becomes higher at a small flow rate due to the increase of the whirl velocity of the flow from the
upstream guide vane, as shown in Figure 13b. This is the reason why the peak of η2 appeared in 10
theof 11
Sustainability 2016, 8, 756
small flow rate in the case θ = 22.5 degrees.
0.6
20
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
C
T
2
0.4
 = 60 deg. Zg = 26
Zr = 30
15
0.2
10
5
 = 60 deg. Zg = 26
Z = 30
r
0
0
0
1
2
3
4
0
1
2

3
4

(a)
(b)
20
 = 60 deg. Zg = 26
Zr = 30
C
A
15
10
 = 37.5 deg.
 = 30.0 deg.
 = 22.5 deg.
5
0
0
1
2
3
4

(c)
Figure 13. Variation of turbine performance due to setting angle of guide vane. (a) Secondary
Figure 13. Variation of turbine performance due to setting angle of guide vane. (a) Secondary
conversion efficiency; (b) Torque coefficient; (c) Input coefficient.
conversion efficiency; (b) Torque coefficient; (c) Input coefficient.
5. Conclusions
This present paper shows the impulse turbine performance under a steady air flow condition,
and the effects of the air chamber length L and the setting angle θ of the guide vane on the energy
conversion efficiency of the fixed OWC-type wave energy converter under the oscillating air flow
condition. The chamber length was changed between the three cases L = 0.5 m, 0.7 m and 0.9 m, and the
setting angle of the guide vane was varied from θ = 30.0 degrees to 22.5 degrees and 37.5 degrees.
In the experiments, the maximum efficiencies of the primary conversion and the secondary conversion
were measured by changing the wave length and the rotational speed of the turbine. The following
concluding remarks are obtained.
(1)
(2)
(3)
(4)
The reflected wave is suppressed by changing the chamber length from L = 0.5 m to 0.7 m,
resulting in the achievement of the high primary conversion efficiency;
Almost the same high generating efficiency is achieved in the two cases of L = 0.7 m and 0.9 m
because the reflection coefficient is equally low;
The effect of the setting angle θ of the guide vane on the primary conversion efficiency is small,
within the range of θ = 22.5 degrees to 37.5 degrees;
The recommended setting angle of the guide vane is θ = 30 degrees in the case of the rotor
inlet/outlet angle γ = 60 degrees.
The authors will confirm the successful electricity generation by replacing the AC synchronous
motor with the generator, along with the improvement of the primary conversion efficiency, in the
near future.
Acknowledgments: This investigation was carried out under the “Program for the Promotion of New Energy
Infrastructure Development”, supported by the Mitsubishi Research Institute (MRI)/the Ministry of Economy,
Trade and Industry (METI), Japan. It was also supported by a research grant of the Hatakeyama
Culture Foundation.
Author Contributions: Yasutaka Imai, Shuichi Nagata, Manabu Takao and Toshiaki Setoguchi conceived and
designed the experiments; Tengen Murakami performed the experiments and wrote the paper.
Sustainability 2016, 8, 756
11 of 11
Conflicts of Interest: The authors declare no conflict of interest.
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